1. Field of the Invention
This invention relates to a control device for an internal combustion engine for a vehicle, which is designed to control the internal combustion engine in accordance with air intake quantity which is detected by an air intake quantity detecting means provided in the suction system of the internal combustion engine.
2. Description of the Prior Art
In a conventional control system for an internal combustion engine, the amount of air sucked into the engine is measured by means of an air intake quantity sensor (heat-wire type, Karman's vortex type or a vane type) arranged in the suction pipe. The engine operation is controlled by determining the amount of fuel to be supplied to the engine, the ignition timing, etc., in accordance with the air intake quantity thus measured.
The problem with this type of control device is that, when the throttle valve is almost fully open, the degree of intake air pulsation in the suction pipe is intensified, resulting in an increase in error in the measurement of the intake air amount by the intake air amount sensor, so that sufficient accuracy in measurement cannot be assured. When the fuel amount is determined in accordance with an intake air amount signal containing many errors, the air-fuel ratio of the air-fuel mixture will become either too rich or too lean.
In view of this, a conventional control device is so designed that, whenever the opening of the throttle valve is detected to exceed a predetermined degree, it ceases to utilize the intake air amount signal. Instead, it then estimates the intake air amount from the engine speed, executing the above control in accordance with the air intake quantity thus estimated (see Japanese Unexamined Patent Publication No. 55-142942).
However, the prior art device disclosed in the above-mentioned publication is not free from any problem. As described above, this prior art device is so designed that, when that is engine operated under a condition where it is subjected to generation of intake air pulsation, for example, when the throttle valve is almost fully open, the air intake quantity is estimated exclusively from the engine speed, or more specifically, the basic injection timing is read out from a table in which data is previously stored. If a vehicle equipped with this control device is used even in a place where the air density is relatively low and, consequently, the weight of air contributing to combustion is relatively small, which is the case, for example, on a high altitude, a predetermined basic injection timing which is the same as that used at lower altitudes will be set, and the same amount of fuel as is appropriate for low altitudes will be supplied to the engine, which will result in an excessively rich air-fuel ratio, reduced output and malfunction of the engine.